1. The Field of the Invention
This invention relates to a slow speed wheel composition. More precisely, this invention relates to an improved thermoplastic slow speed wheel composition that has excellent wearing and riding properties and, by virtue of being a thermoplastic, is easily and quickly processed into finished parts for use in devices designed for slow speed operation.
2. Description of the Prior Art
Suitable slow speed wheel compositions should be wear resistant, have resiliency, bounce and offer a "good ride." Since a "good ride" is subjective and will vary from individual to individual, such a wheel composition should be modifiable to suit individual needs. It should come in a variety of hardness ranges and should react well to imperfections in the riding surface. For example, a small pebble should not eject a rider from a skateboard, nor should a small pebble cause a shopping cart wheel to stop rolling. In addition, the surface properties of the wheel should be modifiable for different situations. For example, in some instances a rider of a slow speed recreational wheel may want a slippery wheel so that tricks involving sliding can be performed with ease. Alternatively, the rider of a recreational wheel may want a soft ride wheel for bumpy surface riding or may want good grip when enhanced control is needed. A tired traveler pulling luggage through a airport may want good vibrational damping on their suitcase with wheels.
Wheel compositions suitable in some aspects for slow speed applications have been around since early man. Perhaps the first wheels were constructed of stone. The first skate wheels generally where constructed of wood and had poor wear resistance. In the early 1950s, skate wheels where constructed of metal which had better wear resistance, but reacted poorly to street pebbles and tended to slide. In the mid 1960s, clay wheels where constructed which exhibited enhanced wear over metal and also offered a better quality ride and reacted better to riding surface imperfections such as pebbles. A major innovation took place in the late 1960s with the introduction of crosslinked polyurethane wheels, also known in the art as cast urethane wheels. Such polyurethane wheels exhibit excellent wearing and riding properties. This very brief history of skate/skateboard wheels is summarized in TABLE
TABLE 1 ______________________________________ APPROXIMATE YEAR OF INTRODUCTION OF SKATE/SKATEBOARD WHEEL WHEEL MATERIAL ______________________________________ Late 1880s WOOD Early 1950s METAL Mid 1960s CLAY Late 1960s URETHANES ______________________________________ Source: Transworld Skateboarding, August 1988, pp 176-187.
The vast majority of commercial high performance recreational wheels on the market today are crosslinked polyurethanes, typically formed from the reaction of multifunctional isocyanates and polyols. Varying isocyanates and polyols and varying proportions give wheels of differing hardness ranges, wear resistance and riding attributes. This process produces outstanding wheels with excellent wearing and riding properties. These materials are generally described in Polyurethane Handbook by Guunter Oertel (1993) and by various manufacturers such as Innovative Polymers, Olin Corporation, Bayer Corporation and Kryptonics. They are also described by the Polyurethane Manufacturers Association. This process suffers from several major difficulties. First is a slow set up time of the crosslinked plastic. That is, the time that is required for nearly complete chemical reaction is long and can be from 30 minutes to many hours. Prematurely removing a wheel from a mold can cause dimensions of said wheel to change. Second, a major drawback is the general toxicity of the starting materials (see Guunter, 1993 described above). Great care must be taken to ensure worker safety and to protect the environment. Thirdly, by virtue of the fact that these polymers are crosslinked, they cannot be re-melted and reused for secondary articles.
The driving force for the development of thermoplastic elastomers in the early days was as a possible vulcanized rubber replacement. Many attempts have been made to prepare high performance wheels using conventional thermoplastic processing techniques such as injection molding. Most have not enjoyed commercial success because it comes down to material. Most materials are not suitable for making thermplastically processed wheels. Quite simply, all materials are not created equal.
Low cost, beginner wheels such as those marketed by Variflex.RTM. are injection molded from a variety of plastics. However, these do not have the properties needed for high performance wheels.
Klamer (U.S. Pat. No. 4,699,432) discloses an injection molded dual material safety wheel produced of a rigid material and a soft material wherein the rigid material provides support and the soft material provides shock absorbing characteristics. The dual material wheel is also claimed to be esthetically pleasing. For the rigid material, polyvinylidene chloride and nylon are suggested. For the soft material, polyurethanes are suggested. It should be noted that polyvinylidene chloride, nylon and polyurethanes are each a family of polymers with sometimes vastly different properties and that the inventor does not disclose which type(s) would be suitable. The inventor does not disclose or teach how to deal with adhesion issues associated with said rigid and soft layers and does not teach how to deal with the issues surrounding gating, venting and sink. Most importantly, the author does not suggest the use of main-chain linear copolyetherester elastomers of the present invention.
Krishnan (U.S. Pat. No. 4,669,517) discloses a urethane-rubber composite wheel. No suggestion of using the copolyetherester elastomers of the present invention is given.
Heitfield (U.S. Pat. No. 4,070,065) discloses a molded plastic wheel formed with a thin annular grove. Polyurethane is the suggested material for such a wheel. No suggestion of using the copolyetherester elastomers of the present invention is given.
Hechinger (U.S. Pat. No. 4,208,073) discloses a composite skateboard or skate wheel containing both a high and low coefficient of friction material. The material for the lower coefficient of friction is suggested as urethane and the material with a higher coefficient of friction is suggested as rubber. No suggestion of using the copolyetherester elastomers of the present invention is given.
Williams (U.S. Pat. No. 4,040,670) discloses a injection molded two component wheel composed of a rigid core that is overmolded by a soft material. Preferred materials for the core are cellulose acetate butyrate and preferred materials for the softer outer layer are polyurethanes. No suggestion of using the copolyetherester elastomers of the present invention is given. Furthermore, the suggested polyurethane does not appear to work adequately as a slow speed wheel composition.
Hoeschele (Polymer Science and Engineering, No. 12, Vol. 14, p. 848, 1974) broadly discloses copolyetherester elastomers wherein the polyether is polytetramethylene ether glycol. He further suggests that that they may be useful for tubing, sheet and roto-molded tires. He does not suggest the copolyetheresters of the current invention. The suggested copolyetherester does not appear to work adequately as a slow speed wheel composition.
Wolfe (Rubber Chemistry & Technology No. 4, Vol. 50, p. 688, 1977) discloses structure-property relationships for a variety of copolyetheresters wherein the polyether is polytetramethylene ether glycol, polyethylene glycol, polypropylene glycol and ethylene oxide-capped polypropylene(oxide). Wolfe does not suggest applications for said copolyetheresters.
Brown (Rubber Industry, p. 102, June 1975) broadly discloses copolyetheresters wherein the polyether is polytetramethylene ether glycol. He also discloses blends of said copolyetheresters with polybutylene terephthalate (PBT) and polyvinyl chloride. In addition, he suggests that the said copolyetheresters may be useful in autos, trucks, mobile homes, construction and off-road equipment and utility and recreational vehicles. He further suggests tubing, hoses, wire and cable jacketing, seals and gaskets. He does not suggest using said copolyetheresters in slow speed wheels. The suggested copolyetherester (based on tetramethylene ether glycol) does not appear to work adequately as a slow speed wheel composition.
Copolyetheresters are well known in the art. They can be prepared by known methods or they can be purchased from several manufacturers, including Dupont under the name Hytrel.RTM. or from DSM under the name Arnitel.RTM.. Below in TABLE 2 is much of the art that describes the preparation and some of the suggested uses of copolyetheresters.
Many attempts have been made to prepare high performance slow speed wheels using conventional thermoplastic processing techniques such as injection molding. Most have not enjoyed commercial success because it comes down to material. Most materials are not suitable for making thermplastically processed wheels. Quite simply, all materials are not created equal.
TABLE 2 __________________________________________________________________________ US PATENT NUMBER/PUBLICATION INVENTOR/AUTHOR YEAR DISCLOSED __________________________________________________________________________ 3,763,109 Witsiepe 1973 elastomeric copolyesters 3,766,146 Witsiepe 1973 elastomeric copolyesters 3,651,014 Witsiepe 1972 elastomeric copolyesters 3,723,569 Hoeschele 1973 blends of elastomeric copolyesters with cured epoxy resins Polymer Science No. 42, Cella 1973 morphology of elastomeric copolyester s 727-740 3,801,547 Hoeschele 1974 solid phase polycondensation of copolyesters 3,917,743 Schroeder el al. 1975 elastomeric copolyesters blends 3,963,800 Gipp et al. 1976 elastomeric copolyesters RE 28,982 Crawford et al. 1976 blends of elastomeric copolyesters with polyvinyl chloride polymers 3,718,715 Crawford et al. 1973 blends of elastomeric copolyesters with polyvinyl chloride polymers Rubber Chemistry, Vol. 50, Wolfe, Jr. 1977 structure-property relationsh ips of copolyesters wherein the soft segment is derived No. 4, 688-703 from polytetramethylene ether glycol, PPG, PEO, and EO-PPG 4,124,570 Scheibelhoffer el al. 1978 poly(neopenyl terphthalate/trimelli tate) thermosetting resin 4,205,158 Hoeschele 1980 elastomeric copolyesters wherein soft segment is derived from an ethylene oxide- capped polypropylene glycol; branching agent used 4,251,652 Tanaka et al. 1981 elastomeric copolyesters wherein soft and hard segments have enhanced compatibility 4,328,333 Barbee et al. 1982 elastomeric copolyesters wherein soft segment is derived from a high molecular weight polypropylene glycol 4,687,275 Zeilstra et al. 1987 elastomeric copolyesters wherein soft segment is derived from an ethylene oxide- capped polypropylene glycol; branching agent used 4,970,275 Still et al. 1990 elastomeric copolyesters Poly. Eng. & Sci., Vol. 14, Hoeschele 1974 copolyetheresters wherein polyether is polytetramethylene ether glycol No. 12, 848-852 Rubber Industry, June, Brown 1975 properties of copolyetheresters wherein polyether is polytetramethylene ether glycol 102-106 and blends with PBT and PVC; suggested uses for said copolyeth eresters __________________________________________________________________________ PPG is an abbreviation for polypropylene glycol PEO is an abbreviation for polyethylene oxide EOPPG is an abbreviation for ethylene oxidecapped polypropylene glycol 6?